"You shouldn't be foning so soon," the doctor said as he studied his chart. "Mr. Garson Poole, owner of Tri-Plan Electronics. Maker of random ident darts that track their prey for a circle-radius of a thousand miles, responding to unique enceph wave patterns.

Dick was fascinated by the concept of recognizing people or tracking them via brain patterns - via electroencephalograph. In his 1965 novel The Zap Gun he writes about a door that opens only for the correct brain wave pattern (see Cephalic Pattern Door). Also, in his 1977 novel A Scanner Darkly, he wrote about a Cephalochromoscope (Cephscope), which offered recreational uses for brain waves.

There is a precursor to the idea of an autonomous machine that could track people based on their brain waves; see mechanical cobra from Roger Zelazny's Hugo-awardwinning 1967 novel Lord of Light.